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Process: How Cancer Starts

Cells are continually bombarded by carcinogens (cancer-causing agents such as sunlight and certain viruses). Carcinogens damage specific genes (sections of DNA that control specific cell functions), known as oncogenes, that regulate vital processes such as cell division. Most damaged genes are repaired, but this process occasionally fails. Progressive damage to oncogenes may cause the cell to function abnormally and eventually become cancerous.

Genetic damage

Oncogenes regulate the rate at which a cell divides. They also repair damaged genes and programme faulty cells to self-destruct. In time, carcinogens may cause irreparable damage to a cell’s oncogenes. As damage accumulates, the oncogenes may start to function abnormally, causing the cell to become cancerous. If a faulty oncogene is inherited, a cell may become cancerous much more quickly.

A cancerous cell dividing

In this magnified image, a cancerous cell is dividing to form two cells that contain damaged genetic material.

Carcinogens penetrate the cell and cause repeated damage to oncogenes on the chromosomes. Most newly damaged oncogenes are repaired.

The damage and repair of oncogenes continues. With time, some of the oncogenes in the cell become permanently damaged and cannot be repaired.

If a number of oncogenes controlling key cell functions are permanently damaged, the cell no longer functions normally and becomes cancerous.

Formation of a tumour

A cancerous tumour begins as a single cell. If the cell is not destroyed by the body’s immune system, it will multiply uncontrollably, dividing to form two cells, which in turn divide to form four, and so on. Tumour growth rates are measured by the time taken for the number of cells in a tumour to double (the “doubling time”). The doubling time of a tumour generally varies from about 1 month to 2 years.

Tumour doubling

After only four cell divisions, a cancerous tumour contains 16 cells. The cells double in number regularly, causing the tumour to grow larger.

Cancerous cell

First doubling

Second doubling

Third doubling

Fourth doubling

Tumour size

A solid tumour can usually be detected after 25–30 doublings. At this stage in its growth, a tumour contains about a billion cells and has a diameter of about 13 mm (1/2 in).